The hypothesis that the number of possible states of every finite volume of space-time is finite.

CPT symmetry

It is well known that the universe exhibits CPT symmetry - suggesting that it is reversible under an operation involving inverting charge, parity and time.

A conventional way of looking at this is to consider that the universe would run backwards, if time was reversed - and every particle was replaced by its anti-particle with the opposite charge.

Fredkin has suggested [here] that CPT symmetry is an immediate consequence of T symmetry - i.e. that inverting T has as its consequence inverting the parity and charge of all particles.

I think this is a great way of looking at things. It seem much more likely to be right than the more conventional interpretation - which suggests that the universe does not have simple T-symmetry.

It follows from this that charge and parity are best regarded as some sort of dynamical process associated with individual particles.

To illustrate what sort of dynamical process would have the property of inverting in sign when running backwards, I'll present some diagrams:

  + -

In each of the diagrams the black arrows indicate motion. When these arrows are reversed, the corresponding property with the opposite sign is produced.

If charge and parity work like this, then they will automatically be inverted when the momenta of all particles is reversed.

In the case of charge, it appears likely that charged particles behave like a pumps - with positive charges being sources and negative charges being sinks (or visa versa).

It makes good sense that inverting time would automatically invert the sense in which pumps operate - and would turn sources into sinks - and visa versa.

So, it is no suprise that charge needs inverting when inverting time. The reason why parity is similarly chiral is not so immediately obvious.

Questions remain concerning why there are conservation laws - so that charge appears to be preserved exactly, and parity seems to be preserved almost exactly.

Tim Tyler | |